Method for measuring the active KOH concentration in a KOH etching process

ABSTRACT

The invention relates to a method for in-line measuring the active KOH concentration in a KOH etching process in which process silicon hydroxide is produced by a reduction reaction according to the formula: 2K +  (aq.)+2OH −  (aq.)+2H 2 O+Si→2K +  (aq.)+H 2 SiO4 2−  (aq.)+2H 2  (g). The total concentration of KOH bath is measured by using a refractometer and the measurement result is corrected by the estimated K 2 H 2 SiO 4  concentration.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the nation stage of PCT/FI2006/050436, filed Oct.11, 2006.

BACKGROUND

1. Field of the Invention

The invention relates to a method for in-line measuring the active KOHconcentration in a KOH etching process in which process siliconhydroxide is produced by a reduction reaction according to the formula:2K⁺ (aq.)+20H⁻ (aq.)+2H₂0+Si→2K⁺ (aq.)+H₂SiO4²⁻ (aq.)+2H₂ (g).

2. Related Art

The actual etching reactions in KOH etching are non-trivial complicated.The reaction produces silicon hydroxide in the etching process by areduction reaction. Silicon hydroxide, however, is not a stablecompound, and it is transformed into a silicon complex. One molecule ofhydrogen per one atom of silicon etched is released in the process.

The hydrogen silicate ions H₂SiO4²⁻ in the solution tend to polymerize,which sometimes leads to a ‘slimy’ appearance of the reaction products,and makes determining the actual reactions even more difficult.

For the sake of simplicity the reaction shown above is used here todescribe the .process and the weight/weight concentration values shownlater in the text paper are calculated by using the molar masses ofpotassium hydroxide (KOH, 56.11 g) and dipotassium hydrogen silicate(K₂H₂SiO₄, 172.30 g).

The etch rate of silicon in a KOH bath depends on the bath temperatureand the KOH concentration. Both parameters have to be measured in orderto obtain a sufficiently accurate estimate of the etching time.

SUMMARY

While measuring temperature is a trivial task, there are some challengesin the concentration measurement. As the etching progresses, KOH (andmore importantly, OH⁻ ions) is consumed, as shown in the reactionequation shown above. The byproducts (silicon complexes) do notparticipate in the process, but as the concentrations change, the etchrate changes.

There are measurements, e.g. refractive index measurement, capable ofmeasuring the initial concentration of KOH in water with sufficientaccuracy. However, the reaction products introduce an error into themeasurement, and thus with a single measurement it is not possible todetermine the actual concentration of OH⁻ ions left in the solution.

This error source is not significant with a single etching batch. Ifseveral etching batches are carried out without replacing the KOHsolution, the cumulative error will yield unacceptable results.

If this error is to be compensated for by measurements, the OH⁻concentration has to be measured either directly or indirectly. Theindirect way involves measuring the silicon concentration andsubtracting that from the total concentration, which requires twoindependent measurements.

The economy behind making a reusing or recycling system relies on thehigh cost of obtaining and disposing of chemicals. This cost has to bebalanced with the added cost of the measurement system required inmeasuring the concentration with sufficient accuracy.

The measurement system should also be care-free and require very littlemaintenance apart from possible regular calibration. The system shouldalso be physically small enough to be installed in a process station,and it should measure the concentration in-line, i.e. directly from thebath or at the recycling pump.

As the instrument is used in a semiconductor process, no metal parts maytouch the process liquid. Plastics, ceramics and glasses are allowed.

The concentration range of KOH in typical etching process is between0.15 and 0.4 (15% and 40%) w/w. The amount of pure silicon dissolvedinto the bath is roughly 0.001 w/w (0.1%) per batch. This translatesinto 0.005 w/w (0.5%) of K₂H₂SiO₄.

The problem to be solved can be briefly described so that in the priorart there exists no practical method for measuring the active KOHconcentration in a KOH etching process. Main problems in the prior artmethods are high costs and inaccuracy.

The object of the invention is to obtain a practical method for a methodfor in-line measuring the active KOH concentration in a KOH etchingprocess. The object is achieved with the present invention. Theinvention is characterized in that the total concentration of KOH bathis measured by using a refractometer and the measurement result iscorrected by the estimated K₂H₂SiO₄ concentration.

The invention offers a simple, reliable and sufficiently accuratemeasurement method thus solving the problem of the prior art, i.e.creating a useful method in a situation that no practical method existsin the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in more detail by meansof examples described in the attached drawing, in which

FIG. 1 shows output signals from two different measurement methods in anideal case,

FIG. 2 shows output signals from two different measurement methods in anon-ideal case,

FIG. 3 shows the viscosity and refractive index as functions of KOH andK₂H₂SiO₄ concentrations, and

FIG. 4 shows the K₂H₂SiO₄/KOH ratio and refractive index as functions ofKOH and K₂H₂SiO₄ concentrations.

DESCRIPTION OF THE INVENTION

The background of the invention is described firstly by lookingmulti-component measurements in general. In order to measure allconcentrations in a solution with n components, at least n−1 independentmeasurement results are needed. With a simple binary solution, e.g. pureKOH in water, one measurement is sufficient. In the case of KOH etching,the solution is a tertiary solution with water, KOH and K₂H₂SiO₄. Twoindependent measurement results are required.

If there are two measurement results, a and b which both depend on theKOH and K₂H₂SiO₄ concentrations, the concentrations may be solved byusing the measurement results. However, the two different measurementsmust be different functions of concentrations. In practice, thedifference between the two measurements has to be quite significant togive sufficiently accurate results.

One way to illustrate this is to use equivalue curves. In FIG. 1 the twoaxes represent the concentration of KOH and K₂H₂SiO₄. The output valuesof two different (imaginary) measurement methods (a: dash lines, b:solid lines) are drawn in the picture. All points on a single curverepresent the same output value from a measurement.

In the case of FIG. 1 a reacts mainly to KOH, and there is only a slighterror coming from K₂H₂SiO₄ (increasing concentration of K₂H₂SiO₄increases a). Signal b reacts mainly to K₂H₂SiO₄, and there is only asmall negative error coming from KOH. As the curves are almostperpendicular, a small error in one measurement does not change theresult significantly. For example, if a=25.0 and b=1.40, theconcentrations can be solved to be c(KOH)=0.24 and c(K₂H₂SiO₄)=0.065.Small errors in either a or b do not cause large errors inconcentration.

Unfortunately, most measurements tend to react the same way to bothchemicals; output value increases with increasing concentration ofeither chemical. FIG. 2 depicts this situation. Both a and b have anincreasing value with increasing concentrations. Measurement b is moresensitive to K₂H₂SiO₄ than a, but the sensitivity difference is not verylarge. It is still mathematically possible to solve the concentrationsfrom a and b, but the real-world accuracy is poor. For example, whena=15.0 and b=1.40, the concentrations are c(KOH)=0.14 andc(K₂H₂SiO₄)=0.072. A small change in either a or b changes theconcentrations so much that the additional measurement has very littlevalue.

In practice, the curves are not linear, but the same ideas apply. If thetwo sets of equivalue curves are parallel in some point of the graph,the concentrations cannot be solved. The larger the angle between thecurves is, the more accurate the results are.

The invention is a result of a project in which thoughts were put tolook into the feasibility of active KOH measurement with known processmeasurement methods and their combinations. The most promising methodshave been chosen in the following for closer study.

In the following discussion the measurement methods have been dividedinto physical and chemical properties. The border between these twotypes is difficult to draw, but here the division is such that thephysical measurement methods do not rely on any chemical orelectrochemical reactions in the measurement system itself.

The physical properties of a medium can be divided into optical,thermodynamic and mechanical properties. All of these properties haveassociated measurement methods, which are briefly discussed below.

Optical properties describe how light behaves in a medium. Thedefinition of light is often restricted to electromagnetic radiationvisible to human eye, but in this discussion a broader definition isused; optical measurement methods are methods, which use opticaltechnology. In practice, shorter wave UV (ultra-violet) and longer waveNIR (near-infra-red) and IR (infra-red) instruments are opticalinstruments by their construction.

Optical measurements are reliable and accurate in general. There areseveral possibilities in making the measurement insensitive to aging,and the measurement result is directly in electrical form. Opticalmeasurements are also non-invasive, and the only surface in contact withthe medium is the optical window. Corrosion problems are rare and in awell-designed instrument they can be overcome with suitable materialchoices.

In the case of KOH etching the medium is homogeneous, i.e. its opticalproperties are similar throughout the liquid. There may be hydrogen orair bubbles, but the solution itself is not an emulsion or does notcontain a significant amount of particles. This rules out all turbidity(fogginess) measurements.

There are four different optical properties in a homogeneous medium:refractive index, absorptance, optical activity, and fluorescence.

Refractive index describes the optical density of a medium. Therefractive index is defined as the speed of light in vacuum divided bythe speed of light in the medium. Typical values for aqueous solutionsare between 1.33 and 1.52. Refractive index depends on the compositionof the medium (concentration of its components) and it has a significanttemperature dependence.

The refractive index can be measured with a refractometer. Arefractometer can be successfully used in measuring the concentration ofKOH in aqueous solutions with sufficiently high accuracy (approximately0.1% w/w).

Process refractometers are readily available also as industry-specificsemiconductor models without any metal in the wetted parts (e.g.K-Patents PR-23-M).

A refractometer can be used to measure most binary solutions(two-component solutions), but in multicomponent solutions it is unableto give the concentration of all components separately. On the otherhand, refractometers are insensitive to trace impurities, and thus arewell-suited to the total concentration measurement.

In this specific measurement case, refractive index measurement providesa reliable measurement of the total concentration of KOH and K₂H₂SIO₄but it cannot distinguish between the two chemicals, i.e. therefractometer measurement needs to be complemented by anothermeasurement, which reacts to KOH and K₂H₂SiO₄ in a different way asdiscussed earlier.

The counterpart of refractive index is absorptance. Absorptancedescribes the medium's ability to absorb light. Absorptance is highlydependent on wavelength, and it is sensitive to some trace impurities.

Absorptance can be measured at one wavelength or several wavelengths. inthe most extensive form, the absorptance is measured at a large numberof wavelengths to produce an almost-continuous spectrum. Spectrometry isa powerful analysis measurement, as many chemicals leave a distinctivefingerprint into the spectrum.

It should be noted that as a spectrometer takes a large number ofindependent measurements at different wavelengths, it can, at least intheory, determine all concentrations in a multicomponent medium withoutany complementing measurement.

In practice, spectrometry is not at its strongest when the actualconcentration of different components in a solution is important. Theresponse of measurement signal to the concentration is highly non-linear(exponential), and the measurement range and accuracy are limited.

In the case of KOH etching it would suffice to have a silicon-specificmeasurement with rather moderate accuracy. Unfortunately, the spectralpeaks of K₂H₂SiO₄ are masked by the very strong spectral peaks of water.So, it seems that spectral measurements cannot be used in measuring thesilicon concentration.

It is possible to measure the OH⁻ concentration directly with aspectrometer. For example, ABB manufactures a FTIR (Fourier TransformInfra-Red) spectrometer, which is claimed to be able to measure the OH⁻concentration. However, as the concentration is rather high, themeasurement requires a very short path-length and complicated samplingsystems, and still the accuracy is not very good. Also, FTIR analyzersrequire regular maintenance and carry a high cost of acquisition.

Some media are optically active; they rotate the polarization ofincoming light clockwise or counterclockwise. In the case of KOHetching, there are no optically active molecules present and traditionalpolarimetry cannot be applied.

In some cases even optically inactive liquids rotate the polarizationwhen they are under stress (highly turbulent flow or high flow rates).This tends to be more common in highly viscous liquids. The KOH etchantdoes not exhibit this property to any significant extent.

Fluorescence refers to the property of a medium to emit lower-energyphotons when it is illuminated with higher-energy radiation. Inpractice, many organic molecules emit yellow light when they areilluminated with blue or ultra-violet light. The KOH etchant does nothave this property, at least not enough to use it in a measurement.

Thermodynamically a material has three measurable properties: heatconductivity, heat capacity, and vapor pressure. All of these can bemeasured, and all of them depend on the concentration of dissolvedmaterial.

The heat capacity and conductivity of the KOH etchant do not seem todepend very much on the silicon content. Measuring the heat conductivityand capacity of a liquid is also rather difficult as an in-linemeasurement. These thermodynamic properties do not offer any practicalsolution to the concentration measurement problem.

The vapor pressure of a liquid tends to decrease when impurities aredissolved into the liquid. This decrease of vapor pressure can be seeneither directly in the partial pressure of saturated vapor over theliquid surface or indirectly as an increase in the boiling point ordecrease in the melting point.

There are several possible measurement arrangements to measure the vaporpressure. A single pressure sensor can be used in an evacuated chamberto measure the partial pressure of water directly. The boiling point canbe measured, e.g., by heating the liquid with a suitable chosen power,and the freezing point by using a chilled mirror.

Dissolved K₂H₂SiO₄ decreases the vapor pressure significantly, and thisdecrease could be measured. However, as also KOH decreases the vaporpressure, the effect of K₂H₂SiO₄ is small in the net depression of vaporpressure. As discussed earlier, this is not a desirable property. Inpractice, the behavior of vapor pressure measurement and refractiveindex are so similar that no useful two-component measurement can berealized by their combination.

The KOH solution is visibly more viscous than pure water. The sameapplies to the solution of KOH and K₂H₂SiO₄. This hints to thepossibility of using either viscosity or surface tension measurement todistinguish between KOH and K₂H₂SiO₄. One possibility lies in thedynamic viscosity (viscosity changing as a function of shear rate).

Surface tension measurement from a continuous process does not give veryaccurate results. Even with a stationary sample the surface tensionmeasurement does not give any useful results; K₂H₂SiO₄ does not have asignificantly stronger or weaker effect than KOH on the surface tension.

Viscosity is a highly non-linear phenomenon both as a function ofconcentration and temperature. The viscosity of pure water at roomtemperature is approximately 1 mPas. At 20% KOH the viscosity is 1.6mPas and at 40% the value is approximately 4.0 mPas. So, the viscosityof the etchant is radically different from that of water.

Many viscous liquids are non-Newtonian, i.e. their measured viscositydepends on the shear rate (velocity). Measurements with a high-precisionrotary viscometer did not reveal this behavior either with pure KOH orwith KOH and K₂H₂SiO₄. By these measurements it can be assumed theetchant is a Newtonian liquid, and the dynamic viscosity cannot be used.

Measurements with a capillary viscometer reveal the viscosity behaviorof KOH and K₂H₂SiO₄ is different in the concentration region ofinterest. The effect of K₂H₂SiO₄ is more linear than that of KOH mainlybecause of the narrower concentration range.

FIG. 3 depicts the refractive index (solid line) and viscosity (dashedline) behavior of the etchant. By looking at the angle between thecurves, it seems that there are regions where the two measurements canbe used to solve the concentrations, especially at low end of c(KOH).Unfortunately, at a very important region of interest (aroundc(KOH)=0.32 and c(K₂H₂SiO₄)=0.02) the measurement is impossible as thecurves are parallel.

With chemical and electrochemical measurements, there is a directinteraction between the measurement system and the medium undermeasurement. There are numerous laboratory measurements, which involve achain of reactions to indicate or measure some property of the liquid.In general, these methods are not directly applicable to automatedprocess measurements. Thus, the discussion below is limited toelectrochemical reactions and simple indicator reactions.

There are several electrochemical measurement methods, the simplestbeing conductivity. Conductivity measurement is not a very usefulmeasurement in the case of highly conductive solutions. Due to the highconcentration of KOH the solution has high electric conductance, and thesmall changes due to KOH being transformed into K₂H₂SiO₄ are notsignificant enough. Also, the measurement is not selective, so that bothKOH and K₂H₂SiO₄ increase the conductivity.

A pH measurement is in principle a measurement of active H⁺-ions. As theproduct of H⁺ and OH⁻ ion concentrations is constant in an aqueoussolution, the OH⁻ concentration can be determined from the pH value.However, the pH value of the solution is very high (well above 14), sothat the OH⁻ concentration cannot be determined in practice. Very few pHsensors withstand the environment, and the output signal is logarithmic.

In addition to pH measurement there are some other ion-selectiveelectrochemical measurement methods. No silicon-selective measurementsavailable.

Some chemical properties, e.g. pH, can be indicated by using suitableindicator chemicals. These chemicals can be either free in a solution orbound to some host matrix. Optical indicators are straightforward tomeasure, and indicator molecules do not need any calibration themselves.

It seems, however, that in this application the good properties ofindicators cannot be used. There are no known colour indicators forsilicon, using free indicators would require an automatic titrationsystem, and matrix-bound indicators are unlikely to survive in thecaustic environment.

When seeing through the matters above it can be said that thetheoretical and empirical research carried on the problem of measuringactive OH⁻ in KOH etching bath did not find any practical measurementsolution.

Refractometry provides a robust measurement method for pure KOHsolutions. The downside is that a refractometer gives only onemeasurement point, and the effect of KOH and K₂H₂SiO₄ cannot beseparated. If one concentration is known, the other can be calculatedwith high accuracy.

Spectrometry is the only method providing direct OH⁻ ion concentrationmeasurement. However, the cost of equipment is high, it requires a lotof maintenance, and the accuracy is not as good as required.

All single-variable measurements (e.g., conductivity, viscosity, etc.)give much the same information as refractometry but with clumsiermeasurement arrangements and less satisfactory accuracy. None of themethods under research, apart from refractometry, can reach the 0.1%specification in KOH concentration.

All methods seem to react similarly towards KOH and K₂H₂SiO₄. This makesusing a combination of two different methods impractical.

Despite the difficulties in finding a suitable measurement method astold above, the etch rate can still be determined on-line if one of theconcentrations is known. If the K₂H₂SiO₄ concentration is known,satisfactory results can be obtained even if the concentration is knownwith a relatively low accuracy, as the K₂H₂SiO₄ concentration is muchlower than the KOH concentration. According to the basic idea of theinvention a refractometer will give the total concentration of KOH andK₂H₂SiO₄, and the measurement result is then corrected by the estimatedK₂H₂SiO₄ concentration.

The etching process itself is straight-forward. The amount of potassium(K⁺) does not change in the process. This leaves only two variables; theamount of water, which may change due to evaporation and in thereaction, and the amount of silicon. The amount of silicon can beestimated when the number and type of wafers etched in the solution isknown. The only remaining variable (amount of water) can be measuredwith a refractometer.

It is estimated that one batch of wafers results in 1 g/l of puresilicon dissolved into the bath. When this is translated into K₂H₂SiO₄,the number is roughly 6 g/l or 5 g/kg (0.005 w/w). It can be estimatedthat for ten consecutive batches in the same bath, the concentrationrises up to 0.05 w/w.

As the effect of K₂H₂SiO₄ on the refractive index is similar to that ofKOH, the accuracy required in K₂H₂SiO₄ determination is approximately1:50 of the maximum concentration (an error equivalent to 0.001 w/w ofKOH).

The accuracy in determining the K₂H₂S10₄ concentration described abovecan be reached without any measurements by simple balance calculations.The amount of silicon dissolved from a wafer depends on two factors; thewafer design and the etch depth. The etch depth is highly controlled,and the design parameters (the amount of silicon designed to be removed)can be obtained from the designers.

As the total amount of K⁺ in the bath can be determined from the initialrefractive index and volume, the total amount of K₂H₂SiO₄ and KOH can beestimated at any time during the process. The amount of water maychange, but that does not change the ratio of K₂H₂SiO₄ and KOH. FIG. 4shows the situation graphically. The curves formed by dashed linesdepict the different K₂H₂SiO₄/KOH ratios and the solid line curves arethe equivalue curves for the refractive index. The situation is almostideal, the two sets of curves are almost perpendicular.

The system is slightly prone to cumulative errors. After a certainnumber of batches the estimate of dissolved silicon becomes lessaccurate, which increases the uncertainty in the K₂H₂SiO₄/KOH ratio.However, the method should allow several batches to be etched in thesame etchant without introducing any extra measurements. A realisticestimate of the number of batches is 10.

The invention claimed is:
 1. A method for in-line measurement of anactive KOH concentration in a KOH etching process in which siliconhydroxide is produced by a reduction reaction according to the formula:2K⁺ (aq.)+2OH⁻ (aq.)+2H₂O+Si→2K⁺ (aq.)+H₂SiO₄ ²⁻ (aq.)+2H₂ (g), themethod comprising: measuring a total concentration of KOH and K₂H₂SiO₄in an etch bath using a refractometer, estimating a concentration ofK₂H₂SiO₄ in the etch bath, and determining the active KOH concentrationin the etch bath using the measured total concentration of KOH andK₂H₂SiO₄ and the estimated K₂H₂SiO₄ concentration, wherein theconcentration of K₂H₂SiO₄ is estimated after processing about ten orless batches of silicon and estimating the concentration of K₂H₂SiO₄comprises performing a balance calculation using an estimated amount ofSi dissolved into the bath from each batch.
 2. A method for in-linemeasurement of an active KOH concentration in a KOH etching process inwhich silicon hydroxide is produced by a reduction reaction according tothe formula:2K⁺ (aq.)+2OH⁻ (aq.)+2H₂O+Si→2K⁺ (aq.)+H₂SiO₄ ²⁻ (aq.)+2H₂ (g), themethod comprising: measuring a total concentration of KOH and K₂H₂SiO₄in an etch bath using a refractometer, estimating a concentration ofK₂H₂SiO₄ in the etch bath, and determining the active KOH concentrationin the etch bath using the measured total concentration of KOH andK₂H₂SiO₄ and the estimated K₂H₂SiO₄ concentration, wherein theconcentration of K₂H₂SiO₄ is estimated after etching a number of wafershaving a known design to a controlled etch depth and estimating theconcentration of the of K₂H₂SiO₄ comprises calculating the estimatedamount of K₂H₂SiO₄ in solution from the number of the etched wafers, thedesign of the etched wafers, and the etch depth of the etched wafers.